and internal meshes, and of the minute arteries
and veins of the lung of the frog, renders it impossible
to trace the course of the vessels far
along its surface by the aid of the microscope ;
dipping down along the edges or centre of the
perpendicular meshes, they necessarily pass out
of the focus and elude our view.
The ultimate distribution of the minute arteries
and of the minute veins is thus not easily traced;
but it is obviously such as to admit most readily of
the diffusion of the blood over the various meshes,
external and internal, • by the former, and of its
reception by the latter. A minute artery is generally
seen passing into the centre of each vertical
mesh, whilst this mesh is bordered at its central
edge by a minute vein.
The relative disposition of the large and minute
arteries and veins, and of the external and internal
meshes, is best seen by making a transverse section
of the lung along lines equidistant from the principal
artery and vein. Such a section afforded the
opportunity for taking the sketches seen in Plates
VI. and VII.
Plate VI. represents the artery pursuing its course
along the external surface of the lung, or the external
borders of the vertical meshes. Sometimes this
course is a little obscured by the intervention of a
mesh; it is then denoted by a dotted line.
With this plate, Plate VII. must be compared. It
will then be seen that the large vein, the course of
which is represented in it, occupies the other or
internal borders of the vertical meshes, and is consequently
never obscured by their intervention. In
every part these veins were raised over the pencil
point, before their position was transferred to the
paper.
The minute arteries are frequently seen to terminate,
and the minute veins to originate, in the
central part of one of the meshes, always rather
abruptly. The course of the larger arteries, and
of some of the minute arteries is generally at
the edges of the meshes. Occasionally, an artery
or a vein is observed to traverse the angular
part of a mesh. Whenever a minute artery
is seen, whether terminating or pursuing its
course, innumerable branches of an equal and
uniform size are given off, which are at once
capillaries ; similar capillaries are seen to unite and
form each minute vein. The scene is precisely the
same; it is the course of the globules of the